The subject invention is directed to burnishing tools and methods for burnishing a work piece and more particularly, to new and novel burnishing tools and methods whereby a burnishing tool comprises a tool head having a low friction polymer support bearing.
Ball, wheel, and roller burnishing tools with actively rolling elements are well known, and have been widely used for decades for surface finishing and sizing of metallic articles. Ball and roller burnishing is generally performed in a machine tool, often a lathe, as a final machining operation.
For the purpose of improving the surface finish of a work piece, a rolling element is pressed against the surface of a work piece with sufficient force to plastically deform the high points and asperities or “tooling marks” left from prior machining or grinding. Using a suitable high-hardness rolling element and the appropriate force for the work piece material, elimination of the tooling marks can leave a near mirror finish limited only by the finish of the rolling element. For uniform finishing, the rolling element is pressed against the work piece surface by spring or hydraulic means in a controlled manner.
For precise sizing of a work piece, the rolling element is forced against the surface so that the work piece is deformed slightly to conform to a precise desired dimension. Small reductions in work piece dimensions can be achieved so that the final dimensions conform to tight tolerances. For example, an accurate slip-fit of a piston to a cylinder can be controlled in production using roller burnishing of the cylinder bore and piston diameter. For accurate sizing, the burnishing process is commonly controlled by the interference fit of the work piece with a fixed dimension by the tool's rigidly fixed geometry, so that the finished dimension, and not the burnishing force, is held constant.
Burnishing is a practical and economical means of extending the service life of aircraft, engines, steam and gas turbines, and virtually any component subject to fatigue or stress corrosion cracking (SCC). For surface enhancement, the force with which the rolling element is applied to the surface must be controlled and be sufficient to deform the work piece, thus creating the magnitude and depth of compression desired. Burnishing can also be used to improve the fatigue and stress corrosion cracking resistance of existing components without changing either the material (alloy and heat treatment) or the design (physical dimensions) of components. Processes such as “low plasticity burnishing” described in U.S. Pat. No. 5,826,453, and “deep rolling burnishing” described in U.S. Pat. No. 4,947,668 have been developed to impart a layer of compressive residual stress, with minimal or high amounts of plastic deformation, respectively. Both processes provide compressive residual stress within the surface of a work piece, resulting from a combination of subsurface deformation by Hertzian loading of the rolling element against the surface, and the lateral expansion of the surface by the burnishing tool. Such compressive residual stress operates to retard fatigue crack initiation and propagation, and mitigates stress corrosion cracking by eliminating the necessary tension at the surface.
Burnishing tools used for inducing compressive residual stress in a work piece typically have tool heads comprising of one or more rolling elements supported by either an axel-bearing support or hydrostatic bearings. Axel-bearing tools have wheels or rollers affixed to an axel supported by conventional ball, roller, needle, or simple bronze bearings, with the rim of the wheel contacting the work piece. The multiple cylindrical and conical roller tools commonly used for sizing and finishing use an inner race or shaft to support the rollers retained in a cage. Unfortunately, such tools are bulky, limiting access to the work piece, suffer from continual wear, and develop heat in operation that can limit the speed of burnishing and the tool life.
Burnishing tools have also been developed that have tool heads with hydrostatic bearings to support the rolling element. Ball and roller burnishing using hydrostatic tools provide deep residual compression for surface enhancement to improve fatigue, stress corrosion cracking, and general corrosion performance in a wide variety of applications. Surface enhancement offers great potential for improvements in safety and reduction in the cost of maintenance and repair. Fatigue and stress corrosion cracking can be mitigated in aluminum aircraft structures, landing gear, nuclear components and radioactive waste storage containers, oil and gas drilling and distribution piping, for example.
One such burnishing tool is described in U.S. Pat. No. 4,947,668 which uses a constant pressure hydrostatic bearing in which the burnishing force is controlled by the fluid pressure that also operates to support a rolling element. Other such burnishing tools are described in U.S. Pat. Nos. 5,826,453 and 6,415,486. Such burnishing tools as described above have tool heads containing a hydrostatic bearing that utilizes fluid flow to support the rolling element, and to project outwardly from the tool head to lubricate the rolling element. This fluid can also operate as a coolant as the rolling element contacts and rolls along the surface of a work piece. In such systems, the force of burnishing is controlled by separate hydraulic or other mechanical means. Unfortunately, such burnishing tools must be continuously connected to systems providing a source of the pressurized fluid for supporting and for extending the rolling element outwardly from the tool head. Recovery of the hydrostatic fluid as it is expelled under pressure from the burnishing tool either limits the tool to closed machine tool applications, or requires further complexity by requiring a means of fluid recovery. Hydrostatic bearing tools also have a limited angular range over which the force can be applied to the rolling element. If the processing force is applied to the rolling element at more than a few degrees from the axis of the bearing, the lateral resultant force displaces the fluid separating and extending the rolling element outwardly from the bearing seat. This often results in excessive wear that limits the life of the rolling element. Therefore, hydrostatic bearing tools must be maintained in an alignment nearly normal to the surface of the work piece during the burnishing operation, thereby limiting the range of potential applications.